Measurement devices are now widely known for measuring the flow of various fluids, and such devices have been heretofore suggested for measurement of flow of at least some fluid utilizing acoustic signals.
There is no satisfactory technique now known, however, for obtaining real time measurements of the speed of flow of liquid, and, more particularly, of such measurements in heavily used traffic channels on waterways. While it is presently possible to use an upward looking Doppler system to obtain a vertical profile of the current at one location in a liquid, this will not produce a measurement representative of other points across a broad area such as a water channel.
In water channels, such measurements would heretofore have required cables and other equipment on the sea floor, which is not only expensive, but also is vunerable to breakage, such as can occur, for example, due to dragging anchors and the like. Moreover, measurements that might be obtained through use of current meters moored at points across a channel using radio or other links cannot normally be used because of the traffic hazards created.
An alternative technique using horizontally projected back-scattered sound is also known. For example, sound transmitted from one side of a channel of water whose flow is to be measured is scattered back to a receiver or receivers co-located with the projector. Doppler shifts or horizontal translation of the patterns in the back-scattered sound, or some related effect, may then be detected and the resulting flow inferred (see, for example, A. Laenen and W. Smith, Acoustic systems for the measurement of streamflow, Paper 2213, U.S. Geological Survey, Water-Supply, pages 7 and 20, 1983).
A serious difficulty associated with all such acoustic back-scatter systems however, especially when oriented horizontally, is the fundamental inefficiency of the back-scatter process. Only a minute fraction of the projected sound is returned to the hydrophones. Thus, it is very difficult to achieve measurements over substantial ranges with this technique, in contrast to the forward propagation technique described in this invention in which the projectors point directly toward the hydrophones.
A second difficulty associated with back-scatter techniques is due to scattering from the channel surface or floor, since scattering from these boundaries tends to be much stronger than scattering from particles or inhomogeneities in the water column, and thus imposes severe demands on the acceptable beam patterns of the projectors and receivers. Back-scatter systems are also sensitive to the presence or absence of acoustic scatteres in the water column, and, moreover, it is difficult to tell the depth at which the scattering is taking place.
Normal current measurements in a water channel or the like have therefore been now commonly made over a short time period, and predictions made therefrom using harmonic analysis of the data. Such predictions, however, are subject to significant error from several sources, including meteorological effects such as wind, atmospheric pressure changes, and river run-off, as well as limitations of the current meter measurements themselves.
Measurement of gas flow perpendicular to a transmitted electromagnetic beam is also known, and has been used extensively in connection with the atmosphere. In addition, measurement of gas flow perpendicular to the transmitted beam of an optical arrangement is also known (see, for example, U.S. Pat. Nos. 3,623,361 and 4,201,467).
Measurement, such as flow measurement of water across a channel, cannot, however, normally be made using an electromagnetic beam or an optical arrangement. For measurement of such flow, acoustic waves have therefore been heretofore utilized. When utilizing acoustic signals for measurement of water flow, however, such acoustic devices have heretofore measured the speed of the current in a section oblique to the direction of flow of the water (see, for example, U.S. Pat. Nos. 4,094,193 and 4,446,542).
In addition, while a reciprocal transmission approach has been previously used for water flow measurements in channels, this technique can only be used to infer the flow component along an acoustic beam as opposed to measurement perpendicular to the acoustic paths, which is considered essential in this invention.
The reciprocal transmission approach has been used to measure flow along water channels by the rather complex procedure of setting up acoustic paths at angles to the flow, and the components of flow inferred from the reciprocal travel times may then be combined to deduce the component of flow along the axis.
Aside from the additional complexity of set-up, this technique is subject to serious error due to the required assumptions regarding similarity of the flow field along the separate acoustic paths. When flow measurements along each path are combined to form the mean component along the channel access, the resulting combination will only be an accurate representation of the flow if the components from which it is derived are based on similar flow fields. Since the components are derived from paths that are quite different, this assumption cannot be generally valid.
Measurement of flow using a single transmitter and two receivers has also previously been suggested in an article by the named inventors herein (S. F. Clifford and D. Farmer, "Ocean Flow Measurements Using Acoustic Scintillation", J. ACOUS. SOC. AM., Volume 74 (6), pages 1826-1832, December 1983). In the experiment set forth in this article, parallel acoustic paths and pulsed operation were not utilized (very short path lengths were involved), and assumptions were required with regard to the distribution of the flow profiles along the beam due to the use of a single transmitter.